Brushless DC motors of the known type use optical or magnetic sensors to detect the rotor position, which sensors generate a signal corresponding to the rotor position via downstream evaluation electronics. Said signal is used to perform energization of the motor windings via a converter in a manner synchronous with the rotor position. The rotor position can also be determined by detecting the voltage induced in the motor windings, the back-emf. The zero-crossings of the voltage induced by the rotor field in the respectively non-energized phase are detected. In the case of such controllers, an additional sensor is omitted.
DE 10 2007 040 217 A1 describes a controller for a sensorless electronically commutated DC motor. The zero-crossings of the voltage induced by the rotor field in the respectively non-energized phase are determined. In order to be able to rule out zero-crossings caused by disturbances, the instants for the zero-crossings are calculated in advance and the crossings which can be detected in a time period are evaluated. DE 10 2009 045 247 A1 and EP 822 649 B1 also describe sensorless rotor-position detection.